模式匹配类型 [英] Pattern matching on types

问题描述

是否有很好的方法来编写下面的x是类型t的部分？ （我怀疑我应该使用 Data.Type .Equality ，但我不确定如何）

  f :: a  - > Int 
 g :: b  - > Int 
 
 h :: Typeable t => t  - >也许Int 
 h x = x 
（x是类型a）的情况x  - > Just（f x）
（x是类型b） - > Just（g x）
 _  - > Nothing 
  

后续问题

解决方案

这是为code> Data.Typeable 类型安全转换位的作业。 cast： ：（可键入a，可键入b）=> a - >也许b 将运行时类型信息从 Typeable 字典中从 a 和 b 并比较它们;如果 a 和 b 是同一类型，那么它返回 Just >



因此，使用 cast 和 Maybe code>的另类实例，我们有：



  hx = f< $> cast x 
< |> g< $> cast x 
  

据我所知，没有办法避免重复调用 cast ，因为它们出现在不同的类型中。

Is there nice way to write the following "x is of type t" parts? (I suspect I should be using Data.Type.Equality but I'm not sure exactly how)

f :: a -> Int
g :: b -> Int

h :: Typeable t => t -> Maybe Int
h x = case x of
  (x is of type a) -> Just (f x)
  (x is of type b) -> Just (g x)
  _ -> Nothing

Follow up question

解决方案

This is a job for the "type safe cast" bits of Data.Typeable. cast :: (Typeable a, Typeable b) => a -> Maybe b pulls the runtime type information out of the Typeable dictionaries for a and b and compares them; if a and b are the same type then it returns Just its argument, otherwise it fails.

So, with cast and Maybe's Alternative instance in hand, we have:

h x = f <$> cast x
  <|> g <$> cast x

As far as I know, there's no way to avoid the repetitious calls to cast, since they occur at different types.

这篇关于模式匹配类型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！